Changes

This project converts an MP1 into an MP2 (i.e.: adds the 12V output required for the CPC664 and CPC6128). Why? Originally back in the 80's I owned a CPC464 and bought an MP1 for it (MP2s were much more expensive and I had no need for the extra output), but now being the proud owner of a 6128 and having [[LCD And Plasma TV Solution|converted the MP1 to composite]], I use the MP1 regularly and didn't want the hassle of having a second power supply just for the 12V. So it was time to make an upgrade.

IMPORTANT – READ CAREFULLY! If you choose to mount the circuit in a similar manner as I have, using the 7812 heatsink, it should NOT be fixed directly to the MP1 aluminium heatsink! It must be electrically insulated, by using an insulator between the 7812 and the heatsink and a plastic screw/nut, or similar insulating setup. The reason for this is that the 7812/7805 heatsink is internally connected to pin 2. In our circuit, this is connected directly to ground, as is usually the standard. However, as you can see in the MP1 power circuit diagram above, the MP1 5V circuit has two resistors placed between pin 2 and ground (to slightly raise the voltage above 5V), meaning that the heatsink is not as normally expected at 0V, but is actually floating slightly above 0V at around 0.2V. If the 7812 was connected directly to the heatsink it would effectively short-circuit these resistors and mess up the 5V supply. I used a small piece of PCB with a 3mm hole between the 7812 and the heatsink, with a plastic isolating washer. The 7812 is only running at most at around 30% of its maximum capacity, so it shouldn't ever actually heat up and no heatsink is really required in this case. When all has been mounted and connected, the casing can be re-assembled and tested. Always test the voltage and polarity of the new socket BEFORE you test it with your CPC.